The only way to have default values in core fields is for the fields to auto-populate data from a user profile when you are integrating with Community Builder or JomSocial. Besides that, the only field where you can set a default is Country. As stated in a couple other forum posts:
Locate the file country.htm in /components/com_dtregister.
This contains the list of countries to choose from. Scroll down to find United States. You'll see:
<option value="United States" selected="selected">United States</option>
Remove the code: selected="selected"
Insert the selected value into the option tag for the country you want to be the default option.